In Japan, ISDB-TSB (Integrated Services Digital Broadcasting for Terrestrial Sound Broadcasting) is used for terrestrial digital audio broadcasting. In ISDB-TSB, in which a plurality of sets of digital data, such as digital audio data, are simultaneously broadcast, a transmission band as wide as one channel for television broadcasting is divided into thirteen bands (OFDM (orthogonal frequency division multiplex) segments), and information to be transmitted is assigned to each of the OFDM segments. (OFDM segments will hereinafter be referred to simply as “segments”.)
Transmission parameters in ISDB-TSB include:
Segment bandwidth: 432 kHz
Transmission bandwidth: 5,616 kHz (for 13 segments)
Modulation: OFDM
Carrier modulation: DQPSK (differential quadrature phase shift keying), etc.
Multiplexing: MPEG (Motion Picture Image Coding Experts Group)-2 system
Audio coding: AAC (MPEG-2/Advanced Audio Coding) ISDB-TSB broadcasting uses one or three segments. ISDB-TSB broadcasting is planned to use a vacant channel of the current VHF (Very High Frequency) band of television broadcasting.
Part A of FIG. 9 shows an example of frequency allocation for segments, in which an (m−1)-th channel and an (m+1)-th channel are used for television broadcasting while an m-th channel is a vacant channel in television broadcasting. The m-th channel is used for ISDB-TSB broadcasting, and thirteen segments #1 to #13 are transmitted in conjunction using the m-th channel.
In this case, the bandwidth occupied by the segments #1 to #13 is 5,616 kHz (=432 kHZ×13). Thus, in the frequency range of the m-th channel, signals are absent in one of the frequency range lower than the segment #1 and the frequency range higher than the segment #13, or absent in both as shown in part A of FIG. 9.
In a superheterodyne receiver, selectivity characteristics can be readily improved by lowering the intermediate frequency. Accordingly, it is considered to lower the intermediate frequency in an ISDB-TSB receiver. However, lowering the intermediate frequency causes image frequencies to be included in a reception band.
Parts B and C of FIG. 9 show image frequencies in an ISDB-TSB receiver with an intermediate frequency of 500 kHz. As shown in part B of FIG. 9, when receiving a signal of the segment #1, the frequency of a local oscillation signal SLO is set to a frequency that is higher than the center frequency of the segment #1 by 500 kHz. In that case, image frequencies occur in a frequency range shown as hatched in part B of FIG. 9, that is, a frequency range having a width of 432 kHz, centered at a frequency higher than the center frequency of the segment #1 by 1 MHz, which is twice the intermediate frequency. Thus, when receiving a signal of the segment #1, parts of signals of the segments #3 and #4 in the same m-th channel become image interference signals.
Also, as shown in part C of FIG. 9, when receiving a signal of the segment #13, the frequency of the local oscillation signal SLO is set to a frequency that is higher than the center frequency of the segment #13 by 500 kHz. In that case, image frequencies occur in a frequency range shown as hatched in part C of FIG. 9, that is, a frequency range having a width of 432 kHz, centered at a frequency that is higher than the center frequency of the segment #1 by 1 MHz. Thus, when receiving a signal of the segment #13, part of a video carrier signal in the (m+1)-th channel becomes an image interference signal.
Similarly, when receiving signals of the segments #2 to #12, image frequencies occur in a frequency range having a width of 432 kHz, centered at a frequency that is higher than the center frequency of a segment being received by 1 MHz. Thus, when receiving signals of the segments #2 to #12, parts of signals of other segments in the same m-th channel, or part of a video carrier signal in the (m+1)-th channel become image interference signals.
Generally, if an image interference signal is included in a reception signal, a circuit for removing or canceling the image interference signal is provided to improve characteristics relating to image interference. For example, an antenna tuning circuit is provided to remove the image interference signal, or a dual conversion system (double superheterodyne system) is used to remove the image interference signal by a first intermediate frequency filter. Alternatively, a signal having a phase opposite to that of the image interference signal is formed to thereby cancel the image interference signal.
However, in order to remove the image interference signal by the antenna tuning circuit or the first intermediate frequency filter, additional components must be externally attached to an IC, or the circuitry scale must be expanded, resulting in an increased cost. Furthermore, in the case of VHF band as described above, and if image frequencies occur in proximity to the frequencies of a target segment, considerably sharp bandpass characteristics are required for the antenna tuning circuit or the first intermediate frequency filter, which also leads to an increased cost.
When the image interference signal is cancelled using a signal having an opposite phase, high precision is required in the balance of the circuitry, and usually an image attenuation ratio on the order of 40 dB is a maximum value achievable. Thus, if the level of the image interference signal is high, part of the image interference signal remain uncancelled and is output as an interference signal.
FIG. 10 shows a case where three segments are used for ISDB-TSB broadcasting, and it shows a frequency relationship assuming-an intermediate frequency of 1 MHz. Parts A to C in FIG. 10 correspond to parts A to C in FIG. 9.
As shown in FIG. 10, also when receiving signals of the three segments, part of signals of other segments in the same m-th channel, or part of a video carrier signal in the (m+1)-th channel become image interference signals. Thus, also in this case, image interference occurs, and an increased cost is needed in order to avoid the image interference.
The present invention has been made in order to overcome the problems described above.